Electrically conducting materials include metals, such as silver, copper, mercury, and aluminum, ceramics, such as yttrium barium copper oxide, and conducting polymers. Part of the electrical energy is transformed into heat during conduction process. The thermal power loss is potential times current.
Carbon nanotubes (CNTs) are excellent conductors at room temperature. Actually, a single CNT is much better electric conductor than silver at room temperature. However, fabrication of macroscopic material that has good electrical conductance has been difficult. Recently well conducting CNT fiber was fabricated by pulling CNTs out of chlorosulfuric acid solution. That fiber had specific resistance about 20 μΩcm, i.e, more than ten times bigger resistance than silver. Film that was fabricated similarly had ten times bigger resistance than the fiber (Behabtu N, et al., Strong, light, multi-functional fibers of carbon nanotubes with ultrahigh conductivity, Science 339 (2013) 182).
Previously we have found that CNT-xylan nanocomposite is very good conductor (Virtanen J. A, and Moilanen P., PCT/FI2010/000077). However, it was very big surprise to observe that DWNT-xylan may be a room temperature superconductor, or at least has some properties of a superconductor.
Single-, and multiwall carbon nanotubes (SWNT and MWNT) have been dispersed into water using water soluble xylan (Kitamura Shinichi, et al., US Patent Application 20090148573, and US2010224836). Reported conductivities are modest, or low. The present inventors were unable to observe any superconducting properties using SWNT or MWNT with xylan. However, it might be possible to modify xylan so that SWNTs and MWNTs become superconducting, and/or find chiral SWNTs or MWNTs that may become superconducting. Also other types of dispersion agents may be found that will produce superconducting compositions.
Some materials become superconductors, when cooled below critical temperature Tc. Mercury was found to be superconducting 1907 by Kamerlingh Onnes. Tc for mercury is about 4 K that is impractical for industrial purposes. Very slowly Tc was increased to about 23 K with the introduction of niobium alloys. From practical point of view 23 K does not bring great advantage over 4 K, because liquid helium is required for cooling in both cases. However, at the same time the critical magnetic field increased significantly. This allowed the fabrication and use of superconducting magnets.
Next breakthrough came 1984, when Berndoz and Muller discovered lanthanum based cuprate perovskite that has Tc of about 35 K. Other ceramic superconductors have since been discovered, and currently the highest Tc is about 120 K. These superconductors work at liquid oxygen, or nitrogen temperatures that has opened possibility for some interesting applications, such as levitated trains. However, ceramic superconductors have found limited practical use. One reason is that they are fragile. Even after 30 years the superconducting mechanism of these high Tc ceramic superconductors is not well understood, although spin density wave theory is gaining support.
Theoretically two major mechanisms have been proposed for superconductors (M. Tinkham, Introduction to Superconductivity: Second Edition, Dover Books on Physics, 2004, and High-Temperature Superconductivity, Eds. V. L. Ginzburg, and D. A. Kirzhnits). In both mechanisms the charge carrier is a Cooper pair, i.e., two electrons form a pair that does not have spin. Cooper pair is a boson, while single electron is a fermion. Many bosons can occupy the same state, and form a collective wave. In order the two electrons to be bound into a Cooper pair attractive force should be provided. In Bardeen Cooper Schiefer (BSC) theory that force is provided by phonons, i.e., coordinated movement of atomic nuclei. According to BCS theory the upper limit for phononic superconductivity is about 30 K. In 1964 W. Little proposed alternative model, in which the Cooper pair is stabilized by exciton wave in conducting polymers that have polarizable side chains, i.e., electronic movements in the side chains (W. A. Little, “possibility of Synthesizing of Organic Superconductor”, Phys. Rev., 134A (1964) 1416).
Ginzburg, et al., have developed theories about high temperature (40 K<Tc<90 K) and room temperature superconductivity. According to Ginzburg two and quasi two dimensional conducting structures such as thin films, fibers, and tubes could be superconducting at room temperature, if they are surrounded by a thin layer of polarizable material that is strongly coupled with conducting layer. Even then Ginzburg predicted that the highest possible Tc is about 300 K that is just above room temperature.
Occasionally, room temperature superconducting behavior has been claimed, but other scientists have not been able to repeat these results, or so small volume of the sample might have been superconducting that it has no practical importance so far (E. J. Eck, U.S. 61/630,114). Two recent publications are especially relevant in this context, because they claim superconducting domains in graphite in the presence of water (Scheike T., et al., “Can Doping Graphite Trigger Room Temperature Superconductivity?”, Advanced Materials, 24 (2012) 5826), or hydrocarbons (Y. Kawashima, AIP Adv., 3, online May 24, 2013).
The present inventors studied various proposed theories, and possible practical solutions. Although these were useful guidelines, and explain how the present invention was conceived, these theoretical considerations should not be limiting the practical embodiments of the present invention. As a reminder, BSC model was proposed about 50 years after original discovery of superconductivity, and ceramic high Tc superconductivity has not yet been fully explained. The inventors decided to study one dimensional or quasi two-dimensional conductors, as proposed by Little and Ginzburg. Carbon nanotubes seem to be ideal starting point. At room temperature the superconducting electrons should have wavelength of about 1 nm or less. Zig-zag CNTs have an axial lattice constant of 0.246 nm Multiples of this are 0.49 nm, 0.74 nm, and 0.98 nm. Armchair CNTs have an axial lattice constant of 0.426 nm. Twice of that may be important in this context, i.e., 0.85 nm. Fermi wavelength of electrons in CNTs is about 0.8-0.9 nm Superconducting electrons have higher kinetic energy than Fermi surface electrons, and shorter wavelength, although smaller overall energy. Thus, the wavelength of spin density wave might reasonable be 0.74 nm, i.e., epitaxial with zig-zag CNT. In this model each 0.74 nm segment of a CNT is occupied by one Cooper pair (boson) on the average. Also armchair segment 0.85 nm seems to be reasonable, if the Fermi wavelength is near the upper limit 0.9 nm.
Cooper pair should be stabilized by an attractive potential. Little proposes chemically coupled electronically polarizable moieties for this purpose. Chemical bonding with CNTs at required density might destroy electrical conductivity. Ginzburg et al., study also possibility of placing electronically polarizable groups in the vicinity of a conductor without chemical bond. In order to achieve significant polarization, the group should be quite large, perhaps 1 nm or more. If the allowed space along long axis of the CNT is 0.74 nm or less, the polarizable group should be more or less perpendicular against the CNT. This would render the coupling between the CNT and polarizable group weak and ineffective.
Zig-zag CNTs are discussed in more detail. Based on these considerations inventors further conjured that point-like positive charge epitaxially placed at distances of 0.74 nm along the long axis of a CNT would be better alternative than electronically polarizable group for the stabilization of a Cooper pair. Proton, or chemically bonded hydrogen atom is an obvious choice. Hydrogen atom should have a partial positive charge, like in hydroxyl group, in which the charge can be about one fifth of the elementary charge. Positive charges should be immobilized enough so that spin density wave is stable. One way to achieve this is to bind positive charges with polymeric backbone. Polymer should be flexible enough to wrap around of a CNT, but rigid enough so that charged groups do not move too much. Polymer should be so flat, and hydrophobic at least on one side so that it has good interaction with a CNT. Further requirement is 0.74 nm periodicity. Inventors performed molecular dynamics studies, and found that natural compound xylan fulfills all these requirements especially, if the diameter of the CNT is about 2-4 nm. Single walled nanotubes (SWNT) have generally smaller diameter than 2 nm, and multiwalled nanotubes (MWNT) have bigger diameter than 4 nm. Double walled nanotubes (DWNT) have diameters typically between 2-4 nm. Inner wall may also be useful for the conductivity especially, if it has the same chirality than the outer wall. If there are more than two walls, the probability of mismatch increases. In both armchair and zig-zag DWNTs both tubes may have same periodicity along long axis; armchair outer tube (n,n), and inner tube (n-5,n-5); zig-zag outer tube (n,0), and inner tube (n-9,0). These are calculated values based on the spacing of graphene planes in graphite.
Natural xylan has side chains that may not be beneficial for the present invention, although it is supposed that the backbone is against a CNT, and side chains are pointing outward. Xylan can also be produced without side chains (Mortimer J. C., et al., Proc. Natl. Acad. Sci., 107 (2010) 17409), or alternatively side chains may be hydrolyzed at least partially away.
Water soluble xylan has been previously used to disperse CNTs into water. SWNTs and MWNTs were dispersed, and it is possible that DWNTs were not tested. Present inventors do not use water-soluble xylan but instead xylan in slightly acidic milieu. Water-soluble xylan is alkali metal salt of xylan. Both forms of xylan may give equal results.
Similar reasoning may show that cellulose is better suited for armchair CNTs, because separation of cellulose molecules in cellulose crystal is 0.835 nm, i.e., very close to one possible periodicity 0.85 nm of armchair CNTs.
Proving that material is superconducting is very complicated. Resistance should, of course, be zero. Experiments can only prove that resistance is below some very small value. Four point measurement is recommended for accurate resistance measurements. Two point measurements may give a significant apparent resistance, because the energy gap is twice the stabilization energy Δg of Cooper pair, and the voltage over the conducting film and electrode should be able to break the Cooper pair. Thus, two point measurements, or modified four point measurements can give the energy gap Δg. Magnetic effects are equally important than zero resistance, and energy gap for proving superconductivity.